The present invention relates to a cellular radiotelephone system including at least one mobile communication station, and particularly to a transmission power control method for dynamically adjusting transmission power of a base station and a mobile station in a digital cellular radiotelephone system as well as in an analog system.
Generally, it has long been considered that, in a radiotelephone system with at least one mobile station in a communicating pair of stations, it is advantageous to provide for adjustment of transmission power. The stations will then use only the lowest reliable power level and thereby reduce the likelihood of interference among co-channel stations engaged in different calls.
One example of station power control in a cellular radiotelephone system is the Radiotelephone Transmission Power Control contained in U.S. Pat. No. 4,613,990 to Samuel W. Halpern, which teaches a method with a base station dynamically controlling transmission power levels of a base station and a mobile station in an analog cellular radiotelephone system. The dynamic transmission power control method is performed as follows. Initially, if a call is attempted then the base station is required to assign a voice channel to accommodate the call; to do this, the base station searches for an idle channel within its network. If an idle channel is found, the idle channel is allotted to the mobile station. Next, a received signal strength indicator (RSSI) of the base station according to the transmission of the mobile station is periodically measured. The measured RSSIs are averaged within a predetermined time and the averaged value is compared with predetermined maximum and minimum threshold amplitudes to check whether the average value is within a desired range. If the averaged RSSI is smaller than a minimum threshold amplitude, a power increasing message is sent to the mobile station, which then increases its transmission power. At this time, if the mobile station is already transmitting at its maximum power, the mobile station is handed-off during the subscriber's radiotelephone conversation unto an adjacent base station. If however, the averaged RSSI is larger than a maximum threshold amplitude because the mobile station is transmitting at an unnecessarily strong power, a power decreasing message is sent to the mobile station to decrease its transmission power. At this time, if the mobile station is already transmitting at its minimum power, its current state of transmission power is maintained and the subscriber's radiotelephone call is processed.
The base station also adjusts the amplitude of its transmission power by RSSI measurement at the mobile station according to the transmission of the base station. In Halpern '990, the power adjustment is made according to an average of determined threshold amplitude levels, and the transmission of power is adjusted to be at this average. I have observed however, that a problem exists because if the threshold amplitude levels are changed, the average also changes and, after a predetermined number of adjustments, no further increase or decrease in adjustment of transmission power can be made; this consequently results in hand-off of the subscriber's radiotelephone call to another cell site. The Method And Apparatus For Adjusting The Power Of A Transmitter described by Loh J. Feei in U.S. Pat. No. 5,220,678, contemplates a simplistic alternative transmission power adjustment in which one subscriber transmits a decreasing radio frequency power signal starting at a maximum amplitude level. A second subscriber monitors that signal and, when the amplitude of the radio frequency drops to a minimum level required for reliable communication, the monitoring subscriber transmits an acknowledgement.
The Microcell System In Digital Cellular by Wm. C. Lee, U.S. Pat. No. 5,243,598, assigns different sets of transmission frequency channels to a plurality of contiguous cells of a cellular telephone system to arrange for hand-off control in order to maintain continuous communication with mobile radiotelephones moving between cells. Multiple access is achieved with assignment of at least one of the frequencies in the assigned set of transmission frequencies to more than one mobile radiotelephone, with either frequency division multiple access, time division multiple access, or code division multiple access.
The Cellular Mobile Telephone System And Method of U.S. Pat. No. 4,829,554 to Barnes et al. provides a cellular telephone system and method in which channel assignments and radiotelephone call routing are controlled by a central control station and each communication channel is separately controlled at a cell station site. Barnes '554 uses signal strength in determining whether to hand-off a call from one cell site to an adjacent cell cite. It seems to me however, that Barnes et al. '554 has a problem because there is no adjustment of transmission power from either the cell site or the mobile unit based on received signal strength to prevent either an unnecessary hand-off to another cell site or to prevent the termination of a call if the strength between the mobile unit and the cell receiving the hand-off is not sufficient to establish communication.
Meanwhile, in the analog cellular radiotelephone system, one radio frequency channel serves one mobile station, but in a digital cellular radiotelephone system having a time division multiple access (TDMA) method, one physical radio frequency channel is divided into time slots corresponding to a plurality of mobile stations and serves all of those mobile stations at the same time. Accordingly, when a plurality of mobile stations sharing a radio frequency channel are at different distances from the base station, in the conventional transmission power control method, transmission power control for those mobile stations should be performed according to time slots.
The Multiples Information Transmission System of U.S. Pat. No. 4,726,020 to Fino et al. discusses multiplexing techniques in response to the detection of free and engaged channels. Many technical problems are encountered in practice of these techniques in an endeavor to adjust transmission power, that are in part attributable to both each and every time slot having several millisecond units of duration, and to the fast response speed that would be required of hardware used to implement these techniques.
Moreover, when a cell plan of a base station is developed based on these foregoing conventional transmission power control techniques, a coverage radius of a radio frequency channel of a base station should be calculated using its maximum transmission power. Accordingly, a frequency allocation efficiency in the base station is as follows. For example, in a twelve-pattern cluster having twenty-six radio frequency channels per base station in an analog cellular radiotelephone system with eighteen decibel carrier/interference (C/I), the frequency reuse distance is as follows: EQU D=6R (when K=12) (1)
In equation (1),
K indicates a frequency reuse pattern that is the number of cells which do not reuse channels, PA1 R is a RF coverage radius, and PA1 D is a frequency reuse distance, i.e., a distance between cells that can reuse the same frequency.
Here, when the frequency reuse value K is varied, the above equation can be expressed as follows: EQU D=3.46R (when K=4) (2) EQU D=4.6R (when K=7) (3) EQU D=7.55R (when K=19) (4)
As shown in equations (1) through (4), there is a limit to frequency reuse according to frequency reuse pattern.
As described above, the conventional transmission power control method has difficulty in controlling transmission power in each time slot in a TDMA cellular radiotelephone system. Also, the frequency allocation efficiency is lowered due to the limit in frequency reuse.